This invention relates generally to a metal melting tool for cutting holes or irregular shapes in a substantially horizontal worksheet made of metal, plastic or the like. The metal melting tool may be a plasma-arc torch of the type disclosed in Brolund et al U.S. Pat. No. 4,063,059. In the preferred embodiment of the invention, however, the metal melting tool is a laser cutting tool of the same general type as disclosed, for example, in Clark et al U.S. Pat. No. 4,201,905.
A typical metal melting tool of the above type (i.e., a plasma-arc torch or a laser cutting tool) includes a tip spaced just above the worksheet and operable to emit downwardly directed energy for melting the worksheet. Holes or irregular shapes are formed through the worksheet as the latter is moved horizontally relative to the tool, such movement usually being produced by bodily shifting the work beneath the tool along coordinate horizontal axes while holding the tool in a horizontally stationary position. When the tool is a laser, a pressurized stream of cutting gas such as oxygen, air or an inert gas is directed toward the tip of the tool. Certain cutting gases such as oxygen coact with the laser beam to form an exothermic reaction and increase the cutting speed and effectiveness of the laser.
In order for the cutting tip to function properly, it is necessary that the tip remain spaced a substantially constant distance above the worksheet as the latter is moved horizontally relative to the tool. Many worksheets, however, are bowed or cambered and thus the upper surface of the worksheet does not lie in a true horizontal plane. To accommodate such a worksheet, the tool is mounted for up and down movement and carries means on its lower end for maintaining the tool at a constant stand-off height relative to the work. In the Clark et al apparatus, for example, the stand-off means is in the form of a collar which encircles the tip of the tool and carries a plurality of rotatable balls. The balls ride on the upper surface of the worksheet and cause the tool to move upwardly and downwardly as necessary to maintain a substantially constant spacing between the cutting tip and the worksheet. Even though the Clark et al patent refers to the balls as being "anti-friction balls", the balls do create some drag on the worksheet and cause the cutting tip to deflect relative to the direction of travel, such deflection impairing the cutting accuracy of the tool and particularly when a sharply curved or angled shape is being cut. In addition, the rubbing action of the balls against the worksheet causes scratches to be formed in certain types of worksheets such as stainless steel sheets.